A general description of multiple antennas (hereinafter referred to as “MIMO”) technology will hereinafter be given as shown below.
MIMO, corresponds to an abbreviation (or acronym) of “Multiple Input Multiple Output, which refers to a method, which has evolved from the currently used conventional method of using one transmission antenna for one reception antenna, that can enhance data transmission and reception efficiency by using multiple transmission antennas and multiple reception antennas. More specifically, instead of relying on a single antenna path in order to receive one whole message, this method applies the technology of collecting a plurality of data segments, which are received from multiple antennas, so as to complete the transmitted data. By using the above-described MIMO technology, the data transmission rate may be enhanced within a predetermined system period, or a system range respective to a specific data transmission rate may be enhanced. More specifically, the MIMO technology corresponds to a next generation mobile communication technology that may be broadly used in mobile communication User Equipments (UEs) and relay stations. This technology is being highlighted as a new technology that can overcome the limitation in data transmission amount in mobile communication, which is already facing a great deal of limitation due the expansion of data communication.
FIG. 1 illustrates the configuration of a general multiple antenna system.
As shown in FIG. 1, when simultaneously increasing the number of antennas in both the transmitting end and the receiving end, unlike when only one of the receiver and the transmitter uses multiple antennas, a theoretical transmission data size (or capacity) increases in proportion to the number of antennas. Therefore, the frequency efficiency may be enhanced outstandingly.
After the theoretical increase in capacity of the MIMO system has been proven in the mid 90's, a wide range of technologies have been under research and development since then, in order to come with actual results of enhancing the data transmission rate. And, among such technologies, some of the successful technologies are being reflected and applied to diverse wireless communication standards, such as the 3rd generation mobile communication standard, the next generation wireless LAN, and so on.
The active research and development related to multiple antennas technologies that have been carried out so far includes research in the aspect of information theory, which is related to calculating the capacity of multiple antennas communication in diverse channel environments and multiple access environments, research in measuring wireless (or radio) channels and creating models of the MIMO system, and research in a time-space signal processing technology for enhancing transmission reliability and enhancing the transmission rate. In other words, many research projects are being carried out is a wide range of aspects.
The MIMO technology includes a “spatial diversity” technique, which may increase transmission reliability by using symbols that have passed through a diversity of channel paths, and a “spatial multiplexing” technique, which may use multiple transmission antennas so as to transmit multiple data symbols at the same time, thereby enhancing the transmission rate. Furthermore, research in developing a method that can adequately yield the advantages of both techniques (spatial diversity and spatial multiplexing) is also one of the most focused field of research.
FIG. 2 illustrates a structure of the MIMO from the transmitting end. As shown in FIG. 2, a MIMO encoder (201) maps L(≧1) number of layers to Mt(≧L) number of streams. Each of the streams is inputted to a precoder (202). A layer may be defined as a coding and modulation path being inputted to the MIMO encoder (201). Additionally, a stream may be defined as an output of the MIMO encoder (201) passing through the precoder (202).
By generating an antenna specific data symbol in accordance with a selected MIMO mode, the precoder (202) may map the streams to the antennas.
A subcarrier mapper (203) maps the antenna specific data t OFDM symbols.
Layer to stream mapping is performed by the MIMO encoder (201). The MIMO encoder (201) corresponds to a batch processor that operates for M number of input symbols at once (i.e., in a single batch). The input respective to the MIMO encoder (201) may be expressed as an M×1 vector, as shown below in Equation 1.
                    S        =                  [                                                                      s                  1                                                                                                      s                  2                                                                                    ⋮                                                                                      s                  M                                                              ]                                    Equation        ⁢                                  ⁢        1            
As shown in Equation 1, Si represents an ith input signal within a single batch of input symbols. The layer to stream mapping of the input symbols is first performed in a space dimension.
First of all, as shown in Equation 2, which is shown below, the output of the MIMO encoder (201) may be expressed as an Mt×NF MIMO STC (Space Time Coding).x=S(s).  Equation 2
At this point, Mt corresponds to the number of streams, and NF corresponds to a number of subcarriers occupied by one MIMO block. Also, x corresponds to the output of the MIMO encoder (201), s indicates an input layer vector, and S(s) represents an STC matrix.
Furthermore may be expressed as a matrix shown below in Equation 3.
                    X        =                  [                                                                      x                                      1                    ,                    1                                                                                                x                                      1                    ,                    2                                                                              …                                                              x                                      1                    ,                                          N                      F                                                                                                                                            x                                      2                    ,                    1                                                                                                x                                      2                    ,                    2                                                                              …                                                              x                                      2                    ,                                          N                      F                                                                                                                          ⋮                                            ⋮                                            ⋱                                            ⋮                                                                                      x                                                            M                      T                                        ,                    1                                                                                                x                                                            M                      T                                        ,                    2                                                                              …                                                              x                                                            M                      T                                        ,                                          N                      F                                                                                                    ]                                    Equation        ⁢                                  ⁢        3            
In an SU-MIMO transmission, an STC rate may be defined by using Equation 4 shown below.
                    R        =                  M                      N            F                                              Equation        ⁢                                  ⁢        4            
In an MU-MIMO transmission, the STC rate for one layer corresponds to 1.
There are three different formats of the MIMO encoder (201)—SFBC (Space Frequency Block Code), Vertical Encoding (VE), and Horizontal Encoding (HE).
In the SFBC (Space Frequency Block Code), the input being inputted to the MIMO encoder (201) may be expressed as a 2×1 matrix, as shown below in Equation 5.
                    X        =                  [                                                                      s                  1                                                                              -                                      s                    s                    *                                                                                                                        s                  2                                                                              s                  1                  *                                                              ]                                    Equation        ⁢                                  ⁢        6            
The MIMO encoder (201) generates an SFBC matrix as shown below in Equation 6.
                    S        =                              S            1                                S            2                                              Equation        ⁢                                  ⁢        5            
At this point, X corresponds to a 2×2 matrix, and the SFBC matrix X occupies two consecutive subcarriers.
In the Vertical Encoding (VE), the input and output of the MIMO encoder (201) may be expressed as an M×1 matrix, as shown below in Equation 7.
                    X        =                  S          =                      [                                                                                s                    1                                                                                                                    s                    2                                                                                                ⋮                                                                                                  s                    M                                                                        ]                                              Equation        ⁢                                  ⁢        7            
At this point, Si corresponds to an ith input symbol of one batch, and, with respect to the vertical encoding method, S1 . . . SM belong to the same layer.
In the Horizontal Encoding (HE), the input and output of the MIMO encoder (201) may be expressed as an M×1 matrix, as shown below in Equation 8.
                    X        =                  S          =                      [                                                                                s                    1                                                                                                                    s                    2                                                                                                ⋮                                                                                                  s                    M                                                                        ]                                              Equation        ⁢                                  ⁢        8            
At this point, Si corresponds to an ith input symbol of one batch, and, with respect to the horizontal encoding method, S1 . . . SM belong to different layers.
Hereinafter, a method for mapping a stream to an antenna will be described in detail.
The mapping of a stream to an antenna is performed by the precoder (202). The output of the MIMO encoder (201) is multiplied by Mi×Nt, w. The output of the precoder is expressed as Mt×Nt matrix, z. The method for mapping a stream to an antenna may be realized by using Equation 9 shown below.
                                                        z              =                            ⁢              Wx                                                                          =                            ⁢                              [                                                                                                    z                                                  1                          ,                          1                                                                                                                                    z                                                  1                          ,                          2                                                                                                            …                                                                                      z                                                  1                          ,                                                      N                            F                                                                                                                                                                                                  z                                                  2                          ,                          1                                                                                                                                    z                                                  2                          ,                          2                                                                                                            …                                                                                      z                                                  2                          ,                                                      N                            F                                                                                                                                                                          ⋮                                                              ⋮                                                              ⋱                                                              ⋮                                                                                                                          z                                                                              N                            t                                                    ,                          1                                                                                                                                    z                                                                              N                            t                                                    ,                          2                                                                                                            …                                                                                      z                                                                              N                            t                                                    ,                                                      N                            F                                                                                                                                              ]                                                                                        =                            ⁢                                                [                                                                                                              W                          1                                                                                                                      W                          2                                                                                            …                                                                                              W                          M                                                                                                      ]                                ⁡                                  [                                                                                                              x                                                      1                            ,                            1                                                                                                                                                x                                                      1                            ,                            2                                                                                                                      …                                                                                              x                                                      1                            ,                                                          N                              F                                                                                                                                                                                                                    x                                                      2                            ,                            1                                                                                                                                                x                                                      2                            ,                            2                                                                                                                      …                                                                                              x                                                      2                            ,                                                          N                              F                                                                                                                                                                                          ⋮                                                                    ⋮                                                                    ⋱                                                                    ⋮                                                                                                                                      x                                                      M                            ,                            1                                                                                                                                                x                                                      M                            ,                            2                                                                                                                      …                                                                                              x                                                      M                            ,                                                          N                              F                                                                                                                                                            ]                                                                                        Equation        ⁢                                  ⁢        9            
At this point, Nt corresponds to a number of transmission antennas, and zj,k corresponds to an output symbol being transmitted through a jth physical antenna over a kth subcarrier.
In case of a downlink, the base station may decide a number of transmission streams (Mt) depending upon the MIMO mode, which the user equipment is to receive, and the base station may also decide a pilot pattern that is to be transmitted. At this point, when the number of streams that are to be transmitted is equal to 1 or 2, a pilot pattern set may be decided by Equation 10 shown below.
A Sounding command IE (Information Element) for multiple uplink transmission antennas is defined. Herein, Sounding means a function that can maintain a consistent level of communication quality, by consistently (or continuously) measuring the transmission performance of many available frequencies and by changing a currently used frequency to a frequency of a better quality, when the quality of the frequency that is currently being used has become degraded. A sounding signal refers to a signal transmitted by the user equipment to the base station in order to perform the sounding function.
First of all, it has not been clarified as to where the sounding channel of each transmission antenna is to be transmitted. Presently, a single unique offset for FDM (Frequency Division Multiplexing) and a single unique cyclic shift for CDM (Code Division Multiplexing) are notified. However, when multiple antennas are supported in the user equipment, the multiple sounding channels are required to be notified. Therefore, when the number of transmission antennas for the sounding of the user equipment is equal to N, N number of consecutive sounding channel indexes may be allocated starting from the offset or cyclic shift value.
Also, the current user equipment transmits the same number of sounding channels as the actual transmission antennas. An antenna power imbalance may occur, for example, by a hand gripping direction or a polarized antenna direction. Therefore, a power imbalance pattern between each transmission antenna may change slowly rather than changing rapidly. FIG. 3 illustrates a graph showing the influence of antenna power imbalance.
As shown in FIG. 3, when the antenna power imbalance is greater than a specific threshold value, a single antenna transmission shows a more excellent performance than two antenna (SFBC; Space-Frequency Block Code) transmission.
In such situation, in order to resolve the problem of antenna power imbalance, an efficient method for controlling antennas that transmit sounding channels is being required.